Certain position-measuring devices are described in a number of documents. They are used for measuring lengths or angles in machines and devices. Such position-measuring devices include a measuring standard, on which one or more code tracks are applied, and a scanning unit, which scans the code tracks and converts path changes and angular changes into electrical signals. In the case of an angle-measuring device, the measuring standard may be a code disk, on which the code tracks are concentrically positioned. In the case of linear measuring devices, the measuring standard may take the form of a ruler.
In addition, certain conventional position-measuring devices, e.g., angle-measuring devices, include, in addition to the code disk for measuring the rotational angle of a shaft, further measuring standards in the form of further code disks, with the aid of which the number of completed revolutions is measured.
These additional code disks are driven via a reduction gear that reduces the speed of the shaft in a suitable manner. Such angle-measuring devices are also referred to as multiturn rotary transducers.
Certain linear measuring devices include rulers that are subdivided into several consecutive position segments, in which the coding of the code tracks cyclically repeats in the measuring direction. In order to count the passed position segments, one or more code disks are provided which are driven via a reduction gear during the scanning of the ruler, in a manner analogous to the multiturn rotary transducer.
Position-measuring devices, which include a plurality of measuring standards, require a complicated mechanical and electrical assembly, which results in high manufacturing costs. In this context, an important criterion is the scanning units for reading the code tracks of the individual measuring standards. In the case of optical scanning, for example, the scanning units include an illumination unit, whose light is modulated by the code track to be read, and a detector unit, which receives the modulated light and converts it into electrical position signals. If separate scanning units positioned to be spatially separated from one another are needed for scanning the individual measuring standards, then this has a considerable influence on the complexity of the position-measuring device.
An example of such a position-measuring device is described in German Published Patent Application No. 28 17 172. This relates to a multistage, shaft-angle encoder having a first angle-increment code disk and a plurality of succeeding angle-increment code disks. The code disks are arranged in two planes, the succeeding angle-increment code disks being together on one plane. All of the code disks are read separately at scanning locations spatially separated from one another. Therefore, a separate scanning unit in the form of a light source and photoelectric cells belonging to it is needed for each code disk. The required mechanical and electrical assembly is highly complex and cost-intensive.
Therefore, in order to achieve a simple and inexpensive design, the number of required scanning units may be reduced. Thus, Japanese Published Patent Application Nos. 11-132792 and 63-237616 describe absolute multiturn rotary transducers having a code disk for ascertaining the angular position of a shaft, as well as a further code disk for counting the completed revolutions of the shaft. The two code disks are connected via a reduction gear, and their code tracks are only scanned at one location. A disadvantage of these arrangements is that only one code disk is provided for counting the revolutions, which means that the number of countable revolutions is limited.
European Published Patent Application No. 1 457 762 describes a device for measuring the position, the displacement, or the rotational angle of an object. It includes three measuring standards in the form of three consecutive code disks, which are coupled via a differential gear unit. The code disks are scanned by a scanning unit that radially covers the code tracks of all of the code disks. However, this document only describes a device for counting run-through, measured-value cycles or position regions. There are no references to position measurement inside the position regions. Thus, it must be performed separately at another location and therefore requires an additional scanning unit.